Adel B. Gougam

Electron Transport in Molecular Wires

It was found that the intermolecular distance between single molecular units, the number of molecules connected in parallel as v as the interaction strength plays an important role for controlling the transport of electrons in molecule wire. 1) The HOMO-LUMO gap (HLG) of one single SAM unit is reduces when increasing N or decreasing d. 2) The (HLG) is a critical parameter for the molecular conductance. When HLG decreases, CG decreases.3) CG depends on d and N of SAMs. CG increases as d increases and decreases N increases.4) the magnitude of the current increases dramatically and CG remains constant as Gamma1 and Gamma2 increase.

Two-dimensional modeling of the back amorphous-crystalline silicon heterojunction (BACH) photovoltaic device

Back Amorphous-Crystalline Silicon Heterojunction (BACH)1 solar cell can be fabricated using low temperature processes while integrating high efficiency features of heterojunction silicon solar cells and back-contact homojunction solar cells. This article presents a two-dimensional modeling study of the BACH cell concept. A parametric study of the BACH cell has been carried out using Sentaurus after benchmarking the software. A detailed model describing the optical generation is defined. Solar cell efficiency of 24.4% is obtained for AM 1.5 global spectrum with VOC of greater than 720 mV and JSC exceeding 40 mA/cm2, considering realistic surface passivation quality and other dominant recombination processes.

Electrical characteristics of graphene wrinkles extracted by conductive Atomic Force Microscopy and electrical measurements on kelvin structures

One of the key deliverables of using graphene as a channel in transistors is to achieve low source and drain parasitic contact resistance. Careful characterization of the surface of graphene is needed in order to improve understanding of the metal to graphene interface. In this paper, we employ conductive Atomic Force Microscope (C-AFM) to characterize the electrical properties of these wrinkles as a function of the applied force. At low forces, the wrinkles are more conductive than flat regions and at high forces the wrinkles have similar conductance as the flat regions. Graphene devices were fabricated and the total resistance of graphene in these devices was measured to be in the range 2Ω to 10MΩ. Additional research is planned to investigate if the wrinkles impact the electrical contact resistance of large area structures.

Amorphous-crystalline silicon heterojunction solar cells formed by the DC saddle field PECVD system: A deposition parameter optimization

The DC Saddle Field PECVD system was used to deposit hydrogenated amorphous silicon (a-Si:H) layers for high efficiency amorphous-crystalline silicon heterojunction (ACSHJ) solar cells. The plasma controlling parameters; including the chamber pressure, gas phase dopant concentration for the p-type a-Si:H (a-Si:H(p+)) emitter, and substrate temperature were varied. The substrate temperature was found to be a critical parameter for the deposition of intrinsic a-Si:H as epitaxial formation can occur with just a temperature increase of 10°C. The processing capabilities have been developed to construct ACSHJ solar cells with 15.5% conversion efficiency for a 4.2 cm2 area.

PECVD deposition and characterization of silicon oxynitride for optical applications

Silicon oxynitride (SiON) films have been found to possess extremely useful properties for optical applications. In optoelectronics, a major advantage of this material is the ability to tune the refractive index from 1.45 to 2.00, allowing designers the flexibility to custom tailor and optimize the refractive index value in the targeted optical device. In addition, its minimum allowable bending radius is much lower compared to other silica materials. This opens up the possibility of miniaturizing integrated photonic systems. Moreover, silicon oxynitride prepared using Plasma Enhanced Chemical Vapor Deposition (PECVD) can be deposited at high growth rates while exhibiting good homogeneity with wide refractive index tuning range making it a well-suited core layer for planar waveguide technologies and microphotonic devices. In this research work, the deposition process and the properties of SiON are discussed. The obtained refractive index as well as the X-ray photoelectron spectroscopy (XPS) analysis are highlighted. Furthermore, FTIR results as a function of the process parameters are presented and their influence on the film properties is discussed.

Review of Interdigitated Back Contacted Full Heterojunction Solar Cell (IBC-SHJ): A Simulation Approach

In the race to higher efficiency Si-based solar cells, several novel configurations have been introduced in the last two decades. One of them is the combination of the heterojunction cell with an intrinsic thin amorphous layer (HIT TM ) and the interdigitated back contacted solar cell (IBC). Lammert et al. proposed the IBC concept in 1975. Years later, the HIT concept was introduced by Tanaka et al. This design combines the advantages of moving all metal contacts to the rear side, hence eliminating shading losses, on the one hand, and on the other leveraging the exceptional passivating properties of a-Si:H thin films to the crystalline Si-based material (c-Si) hence reducing considerably the interface recombination. Both approaches when used separately have proven to show the highest efficiency of Si-based cells on the market at 24 % and 24.7 %, respectively. This article reviews the numerical simulations when using the combined design. It also displays the simulations performed by the authors of this chapter. The effect of different parameters on the cell performance was investigated; c-Si substrate related such as doping level, thickness as well as surface related including front and back surface passivation, and a-Si:H layer properties (intrinsic and doped). We conclude by displaying the effect of the back side design.

Self-aligned single-mask fabrication process for electro-thermal microactuators using ICP-RIE

Advances in the miniaturization of semiconductor devices have been made possible by new methods of microfabrication techniques . These advances have stimulated the birth of Micro Electro Mechanical Systems (MEMS) technology which enable the fabrication of a wide variety of sensing and actuating devices of microscopic dimensions . Of particular interest are thermal microactuators which provide large deflections and are compatible with existing IC technologies. In MEMS technology, a well controlled etching process is critical for the fabrication of structures with specific geometry and properties. Increasing demand for intricate semiconductor devices has fueled and motivated researches to develop high precision micromachining techniques . Inductively coupled plasma- Reactive ion etching (ICP-RIE) is capable of producing features with high aspect ratio as high as 90:1. Taking advantage of the notching effect when making a structure from silicon on insulator (SOI), structure release without the use of HF acid has been demonstrated. We report on the development of a self-aligned single-mask process for the fabrication of released and movable MEMS devices. ICP-RIE was used to realize the structures directly out of single crystal silicon. Applying side wall passivation, controlling the ratio of ion flux and radical flux, smooth etching profile can be obtained with high aspect ratio. No wet etching process is required to release the structures as is the case with SOI wafers. This approach overcomes the stiction limitation associated with wet etching and yields good thickness uniformity over the entire structure. Electrothermal microactuators with integrated microgrippers were designed, fabricated and characterized. harvesters.

Indium tin oxide and the amorphous-crystalline silicon heterojunction

Amorphous silicon-crystalline silicon heterojunctions were prepared using the DC saddle-field plasma enhanced chemical vapour deposition (DCSF-PECVD) technique followed by RF magnetron sputtering of an indium tin oxide (ITO) layer on the nano-thin amorphous film. Depth dependent time of flight secondary ion mass spectrometry (ToF-SIMS) analysis was carried out in order to examine the compositional influence of the sputtered ITO on the underlying amorphous silicon layers. Three samples were analyzed: one, as deposited, a-Si:H/c-Si heterojunction; two, ITO covered a-Si:H/c-Si heterojunction; and three, similar to sample two but now dipped in 10% HCl in order to etch the ITO prior to SIMS analysis. The pre-treatment of the third sample was done to de-couple potential SIMS sputtering-induced implantation of indium, tin, and oxygen in the underlying silicon layers. SIMS analysis shows indium, tin, and oxygen below the surface of the silicon in both the etched and as-deposited samples. AFM analysis of all the samples was also done, indicating that the ITO surface has a high degree of roughness, which could make uniform etching more difficult and could potentially lead to small residual ITO spots on the surface, creating or enhancing the appearance of mixing in the SIMS results for the etched sample.

Amorphous-crystalline silicon interface prepared using DC saddle-field pecvd

The DC saddle field glow discharge method was used to deposit a-Si∶H in order to passivate c-Si surfaces. The process temperature and the thickness of the a-Si∶H films were varied. In addition subsequent annealing of the smaples were studied. Passivation quality of the a-Si∶H overlayers were studied by measuring the effective minority carrier lifetime in the heterostructures as a function of the minority carrier density in the c-Si wafer. These results are then used to model the surface recombination mechanism in our samples. The defect density and the charge density at the interface are inferred which helps us to distinguish between the effects of electric field and chemical passivation at the interface. It is shown that for our intrinsic a-Si∶H samples improvements in surface passivation are directly correlated with the reduction of interface defect density and field effect passivation is minimal. We have achieved surface passivation with effective carrier lifetime ≫ 5 ms for a 40 nm intrinsic a-Si∶H sample deposited at a process temperature of 200 °C. It is also demonstrated that subsequent annealing, at 240 °C, of the samples which were prepared at process temperatures ≪ 240 °C drastically increases the effective lifetime.